1. Field of the Invention
The present invention relates to multi-color electrophotographic machines, and, more particularly, to setting laser power and developer bias in multi-color electrophotographic machines.
2. Description of the Related Art
Toner patch sensors reflect light off of a toner patch to determine how much toner was laid down during the electrophotographic process. The sensor""s voltage signal from reading a toner patch is compared to the sensor signal from reading a bare surface to produce either a voltage difference or a ratio between the two signals. In either case, when the reflectivity of the bare surface changes due to wear or toner filming, the accuracy of the toner patch sensor""s estimates of toner mass per unit area or fused image density is compromised. Toner patch sensors need a reference reflectivity surface to maintain the sensor calibration. Without such a surface, changes in the reflectivity of the intermediate belt or photoconductive drum will corrupt the accuracy of the patch sensor.
Toner patch sensors are used in printers and copiers to monitor the toner density of unfused images and provide a means of controlling the print darkness. In color printers and copiers, the toner patch sensors are used to maintain the color balance and in some cases to modify the gamma correction or halftone linearization as the electrophotographic process changes with the environment and aging effects. Conventional reflection based toner sensors use a single light source to illuminate a test patch of toner and one or more photosensitive devices to detect the reflected light. In most cases the densities of the toner patches are sensed on the photoconductor. Sensing toner patches on photoconductor drums, in a tandem architecture, however, can be an unattractive option since it requires four sensors (one for each photoconductive drum) and there is often little room for such sensors.
Laser power and developer bias operating points that are used in a toner saver mode produce prints which use less toner per page in order to reduce the cost of the printed page. This savings in toner is accompanied by a loss of print quality which is acceptable to some customers. On monochrome laser printers, the toner saver mode operating points can be established by testing and stored in the printer engine code as fixed values when the printer is manufactured. The toner saver operating points depend on the printer model, the cartridge type and the environment. There has been considerable variability in toner savings for a given setting due to variability in cartridges, laser printheads, and environments.
What is needed in the art is a method of compensating for the changes in the reflectivity of the intermediate belt due to wear and toner filming, and which allows the halftone screens to be accurately linearized based on the patch sensor readings.
The present invention provides a method of compensating for changing substrate reflectivity based upon reading a series of special test patches formed by single solid area layers of cyan, magenta, and yellow toners, and test patches formed by superimposing combinations of these patches.
The invention comprises, in one form thereof, a method of calibrating a multi-color electrophotographic machine having an image-bearing surface. Toner patches of cyan, magenta, and yellow solid areas are formed individually and in superimposed combination to form a series of single layer and multi-layer test patches. Light is emitted onto these single and multi-layer test patches. The amount of light that is reflected off of each test patch is measured. Light is also emitted onto a bare section of the image-bearing surface. The bare section has substantially no toner thereon. An amount of light that is reflected off of the bare section is measured. The step of measuring an amount of light reflected off single layer solid area test patches is repeated for a series of laser power and developer bias conditions. A laser power and/or a developer bias is adjusted dependent upon each of the three measuring steps.
Black toner patches become effectively saturated at target densities on the intermediate belt. Saturation occurs when the intermediate belt reflectivity is less than 10-20%. By measuring lower density patches, one can more accurately predict the electrophotographic settings needed to achieve the target density. The reflection data from lower density patches is fit to an exponential form and extrapolated image densities at higher darkness settings are used to select the electrophotographic operating condition.
An advantage of the present invention is that operating points for toner saver modes are calculated with more accuracy due to the closed loop control provided by the toner patch sensor.